Mixer circuit



Aug. 23, 1955 J. M. DE BELL, JR 2,715,219

MIXER CIRCUIT Filed Aug. 31, 1951 INVEN TOR. JOHN MILTON DE BELL JR.

ATTORNEYS United States Patent fice Patented Aug. 23, 1955 MIXER CIRCUIT John Milton De Bell, in, Passaic, N. J., assignor to Allen B. Du Mont Laboratories, Inc., Clifton, N. 1., a corporation of Delaware Application August 31, 1951, Serial No. 244,653

2 Claims. (Cl. 33311) This invention relates to electric circuits for mixing very high frequency alternating currents. It relates in particular to the so-called hybrid ring mixer.

Heretofore hybrid ring mixers for mixing two alternating currents having substantially the same very high frequency have been constructed in a toroidal or rectangular form. Each of these forms is bulky and ill-adapted for having fine adjustments made thereon to obtain the best possible performance.

Therefore, it is one object of this invention to provide an improved mixing circuit.

Other objects are to provide a more compact high frequency mixing circuit, and to provide such a circuit in a form suitable for having fine adjustments made thereon.

Further objects will be apparent after studying the specification together with the drawing in which the only figure is a longitudinal sectional view of the mixer.

The invention comprises a first coaxial transmission line having an electrical length of about three-fourths of the wavelengths of either of two alternating currents to be mixed. The difference in frequency between the two currents must be small compared to the frequency of either current. Surrounding the first transmission line is a hollow conductor which, together with the outer conductor of the first line, forms a second transmission line. The coaxial signal outlets are substantially equally spaced along the outer conductor, one outlet at or near each end thereof, and two more between the outer two, so that the distance between adjacent outlets is approximately one-fourth of a wavelength of the operating frequencies. The outer conductor of each coaxial outlet is electrically directly connected to the outer conductor of the second urn nission line, and the inner conductor of each coaxial outlet is electrically directly connected to the outer conductor of the first transmission line.

The term outlet as used herein is intended to denote either input terminal or output terminal. The generic term must be used since there is no inherent functional distinction between the four components so designated.

The drawing shows one form of the invention in which the coaxial transmission lines consist of linear elements. The center conductor of the first, or inner, transmission line is a conductor 11, which may be hollow and of any cross-sectional shape. In accordance with standard engineering practices and for purposes of simplicity, conductor 11 and the outer conductors of both transmission lines have circular cross-sections in the embodiment shown.

Surrounding conductor 11 and insulated therefrom in fixed spaced relationship is a hollow tubular conductor 12 forming the outer conductor of the first transmission line and the inner conductor of the second transmission line. Surrounding conductor 12 and insulated therefrom in fixed spaced relationship is another hollow, tubular conductor 13 forming the outer conductor of the second transmission line. The conductor 13 is electrically shortcircuited near each end thereof to the innermost conductor 11 by means comprising slideable shorting discs 14 and 16.

Near one end of the conductor 13 is a coaxial signal outlet 17 having the outer conductor 18 thereof rigidly connected, mechanically and electrically, to the conductor 13. The inner conductor 19 of outlet 17 is connected directly to a nearby point on conductor 12. Axially displaced along the conductor 13 from the outlet 17 is a second coaxial signal outlet 20, the outer conductor 21 of which is connected to conductor 13, and the inner conductor 22 of which is directly connected to conductor 12, exactly as is outlet 17. Still further along the conductor 13, a third coaxial outlet 23 comprising an outer conductor 24 and an inner conductor 26 is similarly connected to conductors 13 and 12 respectively. Near the other end of the conductor 13 is a fourth coaxial outlet 27 comprising an outer conductor 28 and an inner conductor 29 similarly connected to conductors 13 and 12 respectively.

It is well known from the theory of hybrid ring mixers that the impedance Z0 of all four outlets may be the same, in which case, the impedance of each of the inner and outer transmission lines should be 2 Other impedance relationships may be used as described in Microwave Duplexes by Smullin and Montgomery, this book being volume 14 of the Radiation Laboratory Series. The proper impedances may be obtained in the well-known way by selecting the ratio of the diameters of the inner and outer conductors of both transmission lines.

The circuit components connected to the mixer are shown in diagrammatic form and comprise a first signal source 31 connected to supply alternating current to the outlet 17, although, as will be described hereinafter, any of the four outlets may be used. A second source 32 of alternating current is connected to the outlet 23. Utilization devices 33, comprising in this illustration, the two sections of a dipole antenna, are connected to the center conductors 22 and 29 of the outlets 20 and 27 respectively.

The operation of the mixer is predicated on the bridge type balance which obtains between the four outlets. Although the two sources 31 and 32 need not supply alternating currents of exactly the same frequency, the two frequencies must be within the bandpass tolerance of the mixer. For example, the video carrier frequency of 609.25 megacycles together with the normal sidebands lying between 608 and 613.75 megacycles may be supplied by source 31; and the aural carrier frequency of 613.75 megacycles with sidebands between 613.7 and 613.8 megacycles may be supplied by source 32. These frequencies are merely illustrative of one way in which the mixer circuit has been used and are not to be considered as limitations of the invention. For these frequencies, the length of the two transmission lines should be approximately 0.368 meter long, corresponding to three-fourths of a wave-length of the median frequency of 611.5 megacycles. The distance between adjacent pairs of outlets such as outlets 17 and 20, or 20 and 23, or 23 and 27 should be approximately 0.123 meter, corresponding to one-fourth of a wavelength of the median frequency.

The signal applied to the outlet 17 is propagated along the outer transmission line, and, simultaneously, by means of the short-circuiting element 14, along the inner transmission line. If proper termination be assumed so that no standing waves are built up in either the inner or outer transmission lines, the phase of the signal at outlet 27 lags the phase at outlet 17 by substantially 270, since the length of each transmission line is substantially threefourths of a wavelength of the signal frequency. On the other hand, the phase of the signal at outlet 20 lags the phase of the signal applied to outlet 17 by only 90, since the electrical distance between outlets 17 and 20 is one one-fourth wavelength along the outer transmission line. The electrical distance from outlet 17 through the inner transmission line and back along the outer line to outlet 20 is one and one-quarter wavelengths which results in effectively the same 90 phase lag.

It will be noted that the signal traveling along the inner transmission line is exactly in phase with the signal traveling along the outer transmission line at outlets 20 and 27.

In the same way, the electrical distance between outlets 17 and 23 is one-half of a wavelength along the outer line and one wavelength along the inner and outer lines. Therefore, due to the phase opposition which obtains, no electrical energy may be withdrawn at the outlet 23, provided, of course, that the frequency of the signal is within the mixer passband. The inverse is likewise true so that the sources 31 and 32 are effectively isolated from each other.

A signal applied by the source 32 to the outlet 23 also splits in the same way as the signal applied to the outlet 17. However, the signals from outlet 23 arrive at both outlets 20 and 27 with the same 90 phase lag and are in phase with each other. Since it is well-known that signals applied to the turn-stile antenna should come from two sources differing in phase by 90, the signals applied to the outlets 17 and 23 may be used for this purpose. Since the signals applied to the outlet 17 may be taken from the outlets 20 and 27 with a phase difference of- 180 and the signals applied to the outlet 23 may be taken from the outlets 20 and 27 in the same phase, these with 0 phase difference between them, it is only necessary to provide a 90 phase shift between either outlet 20 or outlet 27 and the corresponding branch of the turn-stile antenna 33. It is immaterial whether the 90 phase shift device 34 be connected to outlet 20 or- 27 and also whether the phase shift device provides a leading or lagging phase shift.

Although one embodiment of this invention has been shown and described, it will be obvious to those skilled in the art that modifications may be made without departing from the scope thereof as embodied in the following claims.

What is claimed is:

1. An electrical mixer circuit for electrical waves within a band about a predetermined wavelength, said circuit comprising a first conductive member having an effective length substantially three-quarters of said predetermined wavelength; a second tubular conductive member havbeing equal to 4, ing substantially the same eflective length as said first conductive member, said first and second conductive members forming a coaxial transmission line; a third tubular conductive member having substantially the same effective length as said second conductive member and surrounding said second conductive member to form in combination therewith a coaxial transmission line; a conductive connection between said first and third conductive members at both ends thereof; and four coaxial outlets each having the outer conductor thereof electrically connected to said third conductive member and the inner conductor thereof connected to said second conductive member, one of said outlets at substantially each end of said conductive members, the two remaining outlets being substantially equally spaced along said conductive members from each other and from the outlets at said ends.

2. An electrical mixer circuit for electrical waves within a band about a predetermined wavelength, said circuit comprising a first hollow tubular conductive member having an effective length substantially three-quarters of said predetermined wavelength, a second tubular conductive member having substantially the same effective length as said first conductive member and located therein to form a coaxial transmision line therewith; four coaxial outlets each having an impedance Z and each having the outer conductor thereof electrically connected to said first conductive member and the inner conductor thereof connected to said second conductive member, said outlets being spaced substantially one-quarter wavelength apart along said conductive members with an outlet adjacent each end thereof; a third conductive member having substantially the same effective length as said first conductive member and lying within said second conductive, member to form a second coaxial transmission line therewith, an electrical connection between said first and said third conductive members, substantially at the ends thereof, the im pedance of said first and said second transmission lines References Cited in the file of this patent UNITED STATES PATENTS 2,169,305 Tunick Aug. 15, 1939 2,251,997 Goldmann Aug. 12, 1941 2,445,895 Tyrrell July 27, 1948 2,456,770 Dearing Dec. 21, 1948 2,469,222 Atwood May 3, 1949 

